Preparation of polyethylene

ABSTRACT

Ethylene homopolymers and copolymers are prepared in a tube reactor at from 160° C. to 350° C. and pressures in the range from 500 to 5000 bar, with or without use of molar mass regulators, using a peroxide mixture comprising from 1 to 95 mol %, based on the total amount of the peroxide mixture, of at least one cyclic peroxide of the formula I,  
                 
 
     where the radicals R are identical or different and are selected from among alkyl groups and aryl groups, as polymerization initiator.

[0001] The present invention relates to a process for preparing ethylenehomopolymers and copolymers in a tube reactor at from 160° C. to 350° C.and pressures in the range from 500 to 5000 bar, with or without use ofmolar mass regulators, wherein a peroxide mixture comprising from 1 to95 mol %, based on the total amount of the peroxide mixture, of at leastone cyclic peroxide of the formula I,

[0002] where the radicals R are identical or different and are selectedfrom among alkyl groups and aryl groups, is used as polymerizationinitiator.

[0003] The high-pressure polymerization process is a tried and testedprocess for producing low density polyethylene (LDPE) which is carriedout industrially in numerous plants worldwide with great success.Initiation of the polymerization in the high-pressure polymerization isusually effected by means of atmospheric oxygen, by means of peroxides,by means of other free-radical initiators or by means of mixtures ofthese. In practice, it has been found to be particularly advantageous toinitiate the polymerization reaction simultaneously at a plurality ofpoints within the reactor and thus to keep the reactor yield high andthe product quality at a uniformly high level. For this purpose, thefree-radical chain initiators used for initiating the polymerizationhave to be added to the reaction medium in an appropriate manner.

[0004] In order to increase the capacity of existing reactors, effortsare made to achieve very high conversions. However, limiting factors arethe polymerization temperature and the polymerization pressure, whichhave a specific upper limit depending on the product type. In addition,spontaneous ethylene decomposition can occur at above about 350° C.

[0005] Selection of the polymerization initiator enables the conversionto be increased within certain limits. It is desirable to havepolymerization initiators which decompose quickly but can neverthelessbe handled safely. A good method of testing the decomposition rate of apolymerization initiator in the high-pressure process is to record thetemperature profile. For this purpose, the temperature profile in thepolymerization in the high-pressure tube reactor is plotted over thereactor length. Immediately after the first introduction of initiator,the temperature rises steeply due to the enthalpy liberated in thepolymerization reaction and then drops again. After the temperature hasdropped by about 20-100° C., more initiator is introduced and thetemperature again rises steeply and then drops again. This procedure isrepeated a number of times, depending on the number of reaction zones.The decisive indication of the complete reaction of a peroxide is thecooling curve which in the case of complete decomposition is steeperthan in cases in which proportions of the peroxide remain in thereaction mixture even after going through the temperature maximum.

[0006] The reaction is generally carried out with a plurality ofperoxides of which at least one decomposes at comparatively lowtemperature being metered in initially at the starting point, i.e. atthe beginning of the reactor.

[0007] EP-B 0 813 550 teaches that cyclic peroxo compounds of theformulae P¹ to P³ are particularly well suited to polymerizing styreneor acrylates. Furthermore, it is disclosed that these can also be usedfor the polymerization of ethylene. The radicals R¹ to R⁶ are,independently of one another, hydrogen, C₁-C₂₀-alkyl, C₃-C₂₀-cycloalkyl,C₆-C₂₀-aryl, C₇-C₂₀-aralkyl or C₇-C₂₀-alkaryl, where the radicals R¹ toR⁶ may also bear substituents such as alkyl, aryl, alkoxy, aryloxy,hydroxy, carboxyl, hydroxyl, halogen, nitrile or amido.

[0008] It has been found that the conversion when using the mostimportant conventional polymerization initiators is still too low. Themost important conventional polymerization initiators are dibenzoylperoxide, di-tert-butyl peroxide (DTBP), tert-butyl perpivalate (TBPP)and tert-butyl perisononanoate (TBPIN). If the conversion is too low,the economics of the high-pressure process are adversely affected. Whenusing the peroxides of the formulae P¹ to P³, the conversions are stilltoo low for most of these compounds or the amount of initiator requiredto achieve a good conversion is too high.

[0009] It is an object of the present invention to provide a process bymeans of which the conversion in the high-pressure polymerization ofethylene is increased further or the consumption of initiator isreduced.

[0010] We have found that this object is achieved by a process forpreparing ethylene homopolymers and copolymers in a tube reactor at from160° C. to 350° C. and pressures in the range from 500 to 5000 bar, withor without use of molar mass regulators, wherein a peroxide mixturecomprising from 1 to 95 mol %, based on the total amount of the peroxidemixture, of at least one cyclic peroxide of the formula I,

[0011] where the radicals R are identical or different and are selectedfrom among alkyl groups and aryl groups, is used as polymerizationinitiator. R is preferably a linear, branched or cyclic C₁-C₂₀ alkyl ora C₆-C₁₈-aryl. In particular, R is selected from among

[0012] C₁-C₈-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl,n-hexyl, cyclohexyl, n-heptyl or n-octyl, preferably linear C₁-C₆-alkylsuch as methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl or isohexyl,particularly preferably linear C₁-C₄-alkyl such as methyl, ethyl,n-propyl or n-butyl; all three radicals are particularly preferablyethyl; and

[0013] C₆-C₁₄-aryl such as phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl,2-anthryl, 9-anthryl, 1-phenanthryl, 2-phenanthryl, 3-phenanthryl,4-phenanthryl and 9-phenanthryl, preferably phenyl, 1-naphthyl and2-naphthyl, particularly preferably phenyl.

[0014] The preparation of such trimeric ketone peroxides can be carriedout by condensation of the corresponding ketones with hydrogen peroxidein the presence of strong mineral acids and is described in theliterature (for example R. Criegee, in Methoden der organischen Chemie(Houben-Weyl), Vol. 8, p. 46, Georg-Thieme-Verlag Stuttgart 1952 or inEP-A 0 813 550).

[0015] The mixtures used according to the present invention aspolymerization initiator comprise

[0016] A) from 1 to 95 mol % of one or more trimeric ketone peroxides asperoxides decomposing at high temperature, preferably from 2 to 50 mol %and particularly preferably from 3 to 30 mol %, and

[0017] B) from 5 to 99 mol % of one or more conventional polymerizationinitiators, preferably from 50 to 98 mol % and particularly preferablyfrom 70 to 97 mol %,

[0018] in each case based on the total amount of the peroxide mixture.

[0019] The polymerization is initiated in each reaction zone by additionof initiators which decompose into free radicals, with the peroxidemixtures according to the present invention being used in at least onereaction zone. The peroxide mixtures according to the present inventionare preferably used in a plurality of reaction zones or in everyreaction zone. As component B), it is possible to use any polymerizationinitiators such as peroxides, preferably organic peroxides, whichdecompose into free radicals, air or oxygen.

[0020] B) preferably comprises one or more conventional peroxides. Theperoxide mixtures according to the present invention are made up so thatthey comprise at least one peroxide decomposing at high temperature,i.e. it does not decompose until a relatively high temperature isreached, and also at least one peroxide decomposing at intermediatetemperature.

[0021] The distinction between peroxides decomposing at high temperatureand peroxides decomposing at intermediate temperature is made by meansof the temperatures at which the half lives t_(1/2) for thedecomposition are 10, 1 or 0.1 hours; it is most usual to report thetemperature at which the half life is 0.1 hour.

[0022] Peroxides decomposing at low temperature have a half life of 0.1hour at temperatures of less than 100° C.

[0023] Peroxides decomposing at intermediate temperature have a halflife of 0.1 hour at temperatures of from 100 to 140° C.

[0024] Peroxides decomposing at high temperature have a half life of 0.1hour at temperatures above 140° C.

[0025] There is a wide choice of commercially available peroxides, forexample the Trigonox® or Perkadox® products from Akzo Nobel.

[0026] Examples of commercially available peroxides decomposing at lowtemperature are:

[0027] Di(2-ethylhexyl) peroxydicarbonate, tert-amyl peroxyneodecanoate,tert-butyl peroxyneodecanoate, tert-butyl peroxypivalate and tert-amylperoxypivalate.

[0028] Examples of commercially available peroxides decomposing atintermediate temperature are:

[0029] Didecanoyl peroxide,2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tert-amylperoxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butylperoxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butylperoxydiethylisobutyrate, 1,4-di(tert-butylperoxycarbo)cyclohexane asisomer mixture, tert-butyl perisononanoate,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,1,1-di(tert-butylperoxy)cyclohexane, methyl isobutyl ketone peroxide,tert-butylperoxy isopropyl carbonate, 2,2-di-tert-butylperoxy)butane andtert-butyl peroxyacetate.

[0030] Examples of conventional commercially available peroxidesdecomposing at high temperature are:

[0031] tert-butyl peroxybenzoate, di-tert-amyl peroxide, dicumylperoxide, the isomeric di(tert-butylperoxyisopropyl)benzenes,2,5-dimethyl-2,5-di-tert-butylperoxyhexane, tert-butyl cumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)hex-3-yne, di-tert-butyl peroxide,1,3-diisopropyl monohydroperoxide, cumene hydroperoxide or tert-butylhydroperoxide.

[0032] The trimeric ketone peroxides of the formula I can be classifiedas peroxides decomposing at high temperatures.

[0033] The half lives of peroxides are usually determined by one of thegenerally used laboratory methods:

[0034] Firstly, a plurality of ampoules or test tubes containing adilute solution having a concentration c₀ of less than 0.2 mol/l,preferably less than 0.1 mol/l, of the peroxide to be examined areprepared, choosing an inert solvent, i.e. a solvent which does not reactwith peroxides, as solvent; preference is given to benzene, toluene orchlorobenzene.

[0035] These ampoules are thermostatted at a defined temperature. Atdefined time intervals, e.g. 1, 2, 3, 4, 6, 8 hours, an ampoule is takenout, cooled quickly and then analyzed for the residual peroxide contentc_(t). This analysis is preferably carried out titrimetrically. Theresults are evaluated graphically. If the relative concentration isplotted logarithmically against the reaction time, the half life atc_(t)/c₀=0.5 on the ordinate can be read off. To determine thetemperature dependence, this measurement is repeated at varioustemperatures.

[0036] The peroxides B) used are preferably one or more peroxidesdecomposing at intermediate temperature or mixtures of one or moreperoxides decomposing at intermediate temperature and one or moreperoxides decomposing at low temperature.

[0037] As polymerization initiators to be used according to the presentinvention, particular preference is given to peroxide mixturescomprising

[0038] A′) from 1 to 50 mol % of one or more trimeric ketone peroxidesas peroxides decomposing at high temperature, preferably from 2 to 40mol % and particularly preferably from 3 to 30 mol %;

[0039] B′) from 20 to 99% by weight of one or more conventionalperoxides as peroxides decomposing at intermediate temperature,preferably from 30 to 98 mol % and particularly preferably from 40 to 97mol %;

[0040] C′) from 0 to 79% by weight of one or more conventional peroxidesas peroxides decomposing at low temperature, preferably from 0 to 68 mol% and particularly preferably from 0 to 57 mol %,

[0041] where A′), B′) and C′) at up to 100%.

[0042] In the first reaction zone, particular preference is given tousing an initiator mixture of from 1 to 10 mol % of A′), from 40 to 60mol % of B′) and from 40 to 60 mol % of C′), where A′), B′) and C′) addup to 100%. In the following reaction zones, preference is given tousing a mixture of from 5 to 40 mol % of A′) and from 60 to 95 mol %B′).

[0043] The peroxides, which are extremely shock- and impact-sensitive inthe pure state, are advantageously introduced as a solution in aliphatichydrocarbons, for example octane and isododecane. The peroxide mixturesare present in the solutions in proportions of from 5 to 60% by weight,preferably from 15 to 40% by weight. According to the present invention,the polymerization initiator mixtures used according to the presentinvention are introduced in amounts of from 0.5 to 100 mol/t ofpolyethylene produced, preferably from 1 to 10 mol/t and particularlypreferably from 1 to 5 mol/t of polyethylene produced.

[0044] The polymerization is carried out at pressures of from 500 to5000 bar, preferably from 1500 to 3500 bar and particularly preferablyfrom 2000 to 3300 bar. The reaction temperatures are above 40° C. Thereaction temperature is from 150° C. to 350° C., preferably from 250° C.to 330° C. and very particularly preferably from 270° C. to 320° C.

[0045] The process of the present invention can be used both forhomopolymerization and for copolymerization of ethylene with othermonomers, provided that these monomers undergo free-radicalpolymerization with ethylene under high pressure. Examples of suitablecopolymerizable monomers are α,β-ethylenically unsaturatedC₃-C₈-carboxylic acids, in particular maleic acid, fumaric acid,itaconic acid, acrylic acid, methacrylic acid and crotonic acid,α,β-ethylenically unsaturated C₃-C₁₅-carboxylic esters or anhydrides, inparticular methyl methacrylate, ethyl methacrylate, n-butyl methacrylateor tert-butyl methacrylate, methyl acrylate, ethyl acrylate, n-butylacrylate, 2-ethylhexyl acrylate, tert-butyl acrylate, methacrylicanhydride, maleic anhydride or itaconic anhydride and α-olefins such aspropene, 1-butene, 1-pentene, 1-hexene, 1-octene or 1-decene. Inaddition, it is possible to use vinyl carboxylates, particularlypreferably vinyl acetate, as comonomers. The proportion of comonomer orcomonomers in the reaction mixture is from 1 to 45% by weight,preferably from 3 to 35% by weight, based on the amount of ethylenemonomer.

[0046] In addition, the flowing reaction mixture can, according to thepresent invention, contain from 0 to 40% by weight, preferably from 1 to30% by weight, of polyethylene, based on the total weight of themonomers.

[0047] In the process of the present invention, the molar mass of thepolymers to be prepared can be regulated in a customary fashion byaddition of molecular weight regulators. Examples of suitable regulatorsare aliphatic and olefinic hydrocarbons such as pentane, hexane,cyclohexane, propene, pentene or hexene, ketones such as acetone,diethyl ketone or diamyl ketone, aldehydes such as formaldehyde oracetaldehyde and saturated aliphatic alcohols such as methanol, ethanol,propanol or butanol. Particular preference is given to using saturatedaliphatic aldehydes, in particular propionaldehydes, or α-olefins suchas propene or hexene. The molecular weight regulator is preferablymetered into the reaction mixture upstream of the tube reactor. It canalso be metered in together with the polymerization initiator at thevarious points along the tube reactor.

[0048] In a preferred embodiment, polymerization initiator is metered inalong the tube reactor at from 2 to 6 points, particularly preferably atfrom 3 to 5 points, so that from 2 to 6 reaction zones are obtained.Here, the total amount of monomer and, if desired, comonomer ispreferably introduced at the reactor inlet.

[0049] In another preferred embodiment, the tube reactor has at leasttwo reaction zones into each of which additional cold or preheatedmonomer and/or cold or preheated comonomer are/is introduced as freshgas stream before the beginning of each reaction zone. Preference isgiven to at least three successive reaction zones, with thepolymerization having to be restarted in each zone by addition of theappropriate initiators. Reactors suitable for carrying out the processare, inter alia, tube reactors which are provided with a series of inletpipes for the initiator and for the introduction of further amounts ofmonomer.

[0050] Reactors as are described in U.S. Pat. Nos. 4,135,044 and4,175,169 can also be operated using the process of the presentinvention. Here, the tube reactor has a comparatively small tubediameter in each reaction zone from the introduction point for initiatorto the temperature maximum compared to the enlarged tube diameter in thesubsequent cooling zone (from the temperature maximum to the nextintroduction point for initiator). This enables a high conversion to beachieved at a relatively low pressure drop over the length of thereactor.

[0051] The tube reactor is usually provided with a cooled jacket toremove the heat of reaction. Preference is here given to a hot waterjacket which may also be segmented.

[0052] The ratio of length to diameter of the tube reactor is preferablyin the range from 10 000 to 50 000, particularly preferably from 15 000to 35 000.

[0053] In general, the mean residence time of the reaction mixture inthe tube reactor is from 30 to 300 seconds, in particular from 30 to 120seconds.

[0054] After the last introduction of polymerization initiator, thereaction mixture is cooled so as to be able to discharge the productfrom the reactor. The reaction mixture is ejected at the outlet end ofthe tube reactor by means of an appropriate high-pressure let-down valvesystem. After discharge of the reaction mixture, the polymer isseparated from unreacted ethylene and, if appropriate, unreactedcomonomer by depressurization, after which the monomers are generallyrecirculated to the reactor.

[0055] This process of the present invention can be carried out in ananalogous manner with upstream installation of a backmixed reactor.After the polymerization has abated in the backmixer, the polymerizationmixture together with still unreacted monomers is introduced via ahigh-pressure pipe, which may additionally be connected to a heatexchanger, into the tube reactor where the pressure is continued asdescribed above. In general, the mean residence time of the mixture inthe backmixed reactor is from 10 to 100 seconds, in particular from 10to 30 seconds, and the mean residence time in the tube reactor is from10 to 200 seconds, in particular from 10 to 100 seconds.

[0056] The process of the present invention enables ethylenehomopolymers and copolymers having particularly useful properties to beprepared. The polymers can also be prepared in a safe and exactlyreproducible fashion by means of the process of the present invention,without explosive decomposition of ethylene occurring in the reactors.The polymers of the present invention have densities of from 918 to 930kg/m³, preferably from 918 to 926 kg/m³ and particularly preferably from920 to 925 kg/m³. The density can, for example, be influenced by meansof the chain regulators and/or the comonomers. The melt flow index inaccordance with DIN 53 735 (190° C./2.16 kg) is usually less than 50g/10 min, in particular less than 10 g/10 min and particularlypreferably less than 5 g/10 min. Polymers having densities over 918kg/m³ can be prepared at conversions of more than 25% in the mannerdescribed above.

[0057] The process of the present invention makes it possible to achievea significant reduction in the amount of free-radical initiator addedfor the same amount of LDPE (low density polyethylene) produced and thusto operate the high-pressure polymerization in a more economical manner.

[0058] Furthermore, the polymers, in particular LDPE, prepared by theprocess of the present invention are particularly well suited to theproduction of films or film products. Films or film products producedfrom the polymers of the present invention have excellent optical andmechanical properties. Preference is given to films having an elongationat break perpendicular to the machine direction of greater than 750% anda dart drop impact strength of greater than 100. The processability ofthese polymers is also particularly good because the pressure buildup inthe extruder just in front of the die is lower.

[0059] In addition, the process of the present invention has the furtheradvantage that stable reactor operation can be maintained at unusuallyhigh maximum temperatures of up to 320° C. without a tendency fordecomposition to occur.

[0060] The process is illustrated by the examples.

EXAMPLES AND COMPARATIVE EXPERIMENTS

[0061] Examples 1-3 and Comparative Experiments A, B, C and D werecarried out in a tubular reactor having a length of 560 m and a ratio oflength to diameter of 33 000. The polymerization initiators were fed assolutions in aliphatic hydrocarbons directly to the feed points of thetube reactor by means of high-pressure piston pumps. The length of thereaction zones in the reactor was determined by the position of the feedpoints. The oxygen-free ethylene was compressed in a plurality of stagesto the respective reaction pressure, admixed with a molar mass regulatorand fed to the inlet points of the tube reactor. Propionaldehyde wasused as molar mass regulator.

[0062] The heat of reaction liberated in the polymerization was removedfrom the reaction mixture by means of a cooling circuit. The resultingpolymer was separated from unreacted ethylene and other low molecularweight compounds in a customary and known manner in the separatorsinstalled downstream of the reactor and was discharged and pelletized bymeans of an extruder and granulator. Unreacted ethylene was purified ina plurality of stages and recirculated to the suction side of thecompressor. Details may be found in Ullmans Encyclopädie der technischenChemie, Volume 19, pp.169-178 (1980).

[0063] The following abbreviations were used:

[0064] tert-butyl perpivalate: TBPP,

[0065] tert-butyl perisononanoate: TBPIN,

[0066] 3,6,9-trimethyl-3,6,9-triethyl-1,4,7-triperoxonane: TPN,

[0067] di-tert-butyl peroxide: DTBP,

[0068] methyl isobutyl ketone peroxide: MIKP.

[0069] The properties of the resulting polymers were determined by thefollowing methods:

[0070] the melt flow index (MFI) was determined at 190° C. and a load of2.16 kp in accordance with ISO 1133 and the density was determined inaccordance with ISO 1183.

[0071] Blown films were produced from the polymers in a customary andknown manner (cf. Ullmans Encyclopädie der technischen Chemie, Volume11, p.673 (1980)), and the use properties of these were determined bythe following methods:

[0072] light scattering of the films in accordance with DIN 53 490,gloss of the films in accordance with DIN 67 530, elongation at break ofthe films in accordance with ISO 527, the puncture strength of the filmsin accordance with DIN 53 373, the dart drop impact value in accordancewith ASTM D 1709/A. The drawability of the blown film was determined ina customary and known manner during blown film extrusion while the filmwas being drawn off and wound up. Here, the takeoff speed was increasedby 2 m/min in steps of 20 s until the film tore.

[0073] Examples 1-3 according to the present invention using3,6,9-trimethyl-3,6,9-triethyl-1,4,7-triperoxonane (TPN)

[0074] The examples according to the present invention describe thepreparation (Table 1) and the properties (Table 2) of the polymers orthe films prepared therefrom which can be compared directly with thepolymers described in the comparative examples.

[0075] A mixture of tert-butyl perpivalate (TBPP), tert-butylperisononanoate (TBPIN) and3,6,9-trimethyl-3,6,9-triethyl-1,4,7-triperoxonane (TPN) dissolved inisododecane was fed to the inlet point of the tube reactor so as tostart the polymerization. At the second feed point of the tube reactor,at which the reaction temperature had dropped back to 250° C. after thefirst temperature maximum, a mixture of tert-butyl perisononanoate(TBPIN) and 3,6,9-trimethyl-3,6,9-triethyl-1,4,7-triperoxonane (TPN)dissolved in isododecane was introduced. At the third feed point of thetube reactor, at which the reaction temperature had dropped back tobelow 250° C. after the second temperature maximum, a mixture oftert-butyl perisononanoate (TBPIN) and3,6,9-trimethyl-3,6,9-triethyl-1,4,7-triperoxonane (TPN) dissolved inisododecane was introduced.

[0076] No difficulties were encountered in carrying out theseexperiments: the change of product type proceeded quickly and smoothly.The feared explosive decompositions of the reaction mixture atrelatively high temperature peaks of 320° C.-325° C. also did not occur.

[0077] Comparative Examples A, B and C (without3,6,9-trimethyl-3,6,9-triethyl-1,4,7-triperoxonane (TPN))

[0078] 3.2 t/h of ethylene were admixed with 1.0-3.0 l/h ofpropionaldehyde (depending on the MFI), compressed to 3000 bar, heatedto 155° C. in a preheater and fed to the inlet of the tube reactor. Theperoxide TPN used according to the present invention was replaced by theconventional peroxide DTBP in the respective mixture at each inletpoint.

[0079] The peak temperatures in the 1^(st), 2^(nd) and 3^(rd) reactionzones are shown in Table 1 for the individual experiments.

[0080] The resulting ethylene polymer was freed of ethylene and lowmolecular weight impurities in a customary and known manner inhigh-pressure and low-pressure separators, stored in hoppers andprocessed to form blown films.

[0081] Table 1 also gives information on the conversion in the reaction,of the specific consumption of peroxides decomposing at high temperatureand the total peroxide consumption. The physicochemical properties ofthe ethylene homopolymer and the use properties of the blown filmproduced therefrom are summarized in Table 2.

Comparative Example D

[0082] This example was carried out in a manner similar to ComparativeExamples A, B and C, except that a peroxide mixture decomposing at hightemperature (tradename=MIKP) was used in place of DTBP as peroxidedecomposing at high temperature. The results in Table 1 show that, atthe same peak temperature of 303° C., significantly lower polymerdensities (0.9190 g/cm² compared to 0.9240 g/cm² for DTBP and TPN) wereachieved. The product properties are therefore not comparable.

[0083] In addition, a tendency for the reaction mixture to decomposespontaneously was observed when MIKP was used. TABLE 1 PO consumption[mmol/ kg of Peroxides introduced [mol/h] PE] Tmax 1^(st) 2^(nd) 3^(rd)Density MFI Conversion High Experiment [° C.] Init. Init. Init. [g/cm³][g/10 min] [%] temp. Total Ex. A 303 TBPP 0.32 0.9240 0.9 26.7 0.8 2.1TBPIN 0.39 TBPIN 0.19 TBPIN 0.20 DTBP 0.19 DTBP 0.22 DTBP 0.24 Ex. 1 303TBPP 0.40 0.9240 0.8 27.5 0.3 1.7 TBPIN 0.46 TBPIN 0.16 TBPIN 0.16 TPN0.03 TPN 0.03 TPN 0.03 Ex. B 305 TBPP 0.27 0.9240 2.0 28.5 0.8 1.7 TBPIN0.27 TBPIN 0.16 TBPIN 0.14 DTBP 0.27 DTBP 0.19 DTBP 0.28 Ex. 2 310 TBPP0.41 0.9240 2.1 29.1 0.4 1.7 TBPIN 0.47 TBPIN 0.17 TBPIN 0.17 TPN 0.05TPN 0.04 TPN 0.04 Ex. C 315 TBPP 0.27 0.9230 4.0 30.9 1.3 2.2 TBPIN 0.27TBPIN 0.16 TBPIN 0.23 DTBP 0.40 DTBP 0.32 DTBP 0.53 Ex. 3 320 TBPP 0.450.9230 4.1 31.4 0.7 1.9 TBPIN 0.45 TBPIN 0.19 TBPIN 0.19 TPN 0.08 TPN0.08 TPN 0.08 x. D 303 0.9190 0.8 26.7 not able to be determined

[0084] TABLE 2 Ex. A Ex. 1 Ex. B Ex. 2 Ex. C Ex. 3 Temperature [° C.]175 168 173 157 167 148 Machine pressure [bar] 281 240 212 184 186 165Output [kg/h] 22.0 22.0 22.0 22.0 21.0 23.0 Draw-down 1 [μm] 21 20 16 1212 11 Film thickness [μm] 50 51 50 50 50 50 Ult. strength MD [MPa] 31.030.5 27.0 28.9 23.0 25.5 Ult. strength CD [MPa] 22.0 24.6 20.0 21.6 19.019.0 Ult. Elongation MD [%] 223 299 305 486 367 430 Ult. Elongation CD[%] 731 779 710 780 700 780 Byk clarity [%] 93.7 94.1 94.3 95.3 95.494.9 Scattering [%] 16.0 16.6 14.0 12.0 10.0 10.0 Haze [%] 7.0 7.1 7.06.0 7.0 6.5 Gloss 20° [%] 44 42 52 56 65 57 Gloss 60° [%] 93 96 98 106105 99 Dart drop impact [g] 114 147 100 125 90 108 Puncture energy[J/mm] 5.5 5.4 4.4 5.1 3.8 3.8 Blocking force [N/15 mm] 113 70 113 92 59112 COF [%] 106 87 108 118 112 120 Film rating, FN 2.5 2.0 2.6 2.0 3.22.0

1. A process for preparing ethylene homopolymers and copolymers in atube reactor at from 160° C. to 350° C. and pressures in the range of500 to 5000 bar, with or without use of molar mass regulators, wherein aperoxide mixture comprising A) from 1 to 50 mol % of one or moretrimeric ketone peroxides of the formula I as peroxides decomposing athigh temperature

 where the radicals R are identical or different and are selected fromamong alkyl groups or aryl groups, and B) from 50 to 99 mol % of one ormore conventional polymerization initiators, based on the total weightof the peroxide mixture is used as polymerization initiator.
 3. Aprocess as claimed in claim 1, wherein all the radicals R are ethyl. 4.A process as claimed in claim 1, wherein the polymerization initiatorused is a peroxide mixture comprising A′) from 1 to 50 mol % of one ormore trimeric ketone peroxides of the formula I as peroxides decomposingat high temperature, B′) from 20 to 99 mol % of one or more conventionalperoxides as peroxides decomposing at intermediate temperature, and C′)from 0 to 79 mol % of one or more conventional peroxides as peroxidesdecomposing at low temperature, where A′), B′) and C′) add up to 100%.